As nanotechnology produces smaller and smaller circuits and other elements, it becomes more difficult to produce and manipulate the nanometer scale objects. While it is known that it is desirable to automate nano-processes, the precise nature of those processes makes automation difficult, if not impossible for many processes.
The line widths of features achievable in semiconductor processing are significantly less than 100 nm. To control critical processes during fabrication, it is necessary to observe and measure the results of those processes. Scanning electron microscopes (SEM) are used to observe microscopic features, but as the feature size of fabricated structures decreases, the resolution of SEM is insufficient, and it is necessary to observe defects on a transmission electron microscope (TEM). While a SEM can observe a feature on a thick work piece, to observe a sample on a TEM, it needs to be thinned to less than 100 nm so that electrons will travel through the sample. It can be exceedingly difficult to thin a sample to less than 100 nm while ensuring that the feature that requires observation remains in the sample and is not milled away in the thinning process.
An operator will typically observe the sample as it is being thinned, stopping regularly to observe the sample to see whether or not the feature is exposed for observation. When the feature to be observed is exposed on the surface, then thinning is stopped. This can be very time consuming and labor intensive. Moreover, the results are inconsistent from operator to operator because the decision of when to stop is subjective.
U.S. Pat. Pub. 2010243889 of Farber et al. describes a method of endpointing when forming a lamella for viewing on a transmission electron microscope. In accordance with Farber, secondary particles are collected as the ion beam thins the lamella, and the image formed from the secondary particles is used to form a rough image of the cross section. The image is rough because the ion beam impacts the lamella at a glancing angle, and because the secondary particles from deep in the trench are not detected as well as the secondary particles from higher up in the trench.